This invention relates to acoustical flowmeter systems and is particularly directed to an improvement in the acoustical flowmeters of the type described and claimed in the U.S. Pat. No. 4,003,252 entitled "Acoustical Wave Flowmeter" by E. J. DeWath which issued Jan. 18, 1977 and the flowmeter system of the type described and claimed in the U.S. Pat. No. 4,164,865 entitled "Acoustical Wave Flowmeter" by L. G. Hall and R. S. Loveland which issued Aug. 21, 1979.
The invention of DeWath was directed to a flow meter having an unobstructed tubular wall thereby eliminating all impediments to the flow path of the fluid and eliminating all cavities in which debris might collect. The advantages of such a configuration is fully set forth in the DeWath Patent. To measure flow of a selected fluid in the DeWath flowmeter, however, required a calibration for that particular fluid and required a recalibration if the flow of a different fluid was to be measured since the flowmeter was not responsive to changes in fluid species or densities.
The Hall and Loveland invention improved the DeWath flowmeter by providing a flowmeter that measured flow accurately regardless of changes in fluid composition or temperature and by providing a flowmeter with a means for determining a change in velocity of sound of the fluid being measured.
In order to accomplish this, the Hall and Loveland acoustical wave flowmeter system had two spaced apart crystal transducers in the wall of the flowmeter conduit (sometimes called a cavity) to produce ultrasonic acoustic compressions at selected frequencies in the fluid within the cavity. The transducers were alternately switched into a transmit and a receive mode to generate upstream and downstream transmitted and received signals with an automatic means to adjust the transmitted frequencies to compensate for changes in velocity of the acoustic compressions in the fluid caused by changes in fluid composition and temperature. The electronic circuitry involved in the Hall and Loveland flowmeter system include means for measuring and storing signals representing the phase difference between the transmitting transducer signal producing the acoustic compressions and the signal produced by the receiving transducer during each of two successive transmit/receive cycles. Circuit means were provided to determine the difference between the signals representing the two successive phase differences wherein the sign of the difference corresponds to the direction of the fluid flow and the magnitude of the difference corresponds to the rate of fluid flow through the flowmeter. Circuit means were also provided to add the two successive phase difference signals together to obtain a signal proportional to the velocity of sound in the fluid moving through the flowmeter. This latter signal indicated the change in composition of the fluid flowing through the meter.
The Hall and Loveland system utilized a phase lock loop in the receiver/transmitter system and operated from a fixed low frequency clock source, sometimes called a free running clock, for, among other things, alternately turning the transmitter and receiver transducer ON and OFF and for operating other components of the circuitry. In order to accomplish the frequency change due to a change in the sum of the two phase differences of the transmitted and received signals of two successive transmit receive cycles (in order to maintain the energy in the acoustic compressions the same), a voltage controlled oscillator (VCO) of a higher frequency than the system clock frequency was used.
However, with the base for all of the timing (gating) signals being held constant and with the frequency of operation of other parts of the system changing according to the frequency transmitted by the voltage controlled oscillator, the fundamental of the transmitted frequency mixed with a higher order harmonic of the low constant clock frequency. This introduced a beat frequency which resulted in a zero drift output signal from the flowmeter. This was objectionable because it indicated output drifts as much as 50 milliliters per second which would change in magnitude as the the frequency of operation changed. The result of the output zero drift was to indicate a flow when there was none.
The source of error was found to be due to the fact that the signal integrator, which immediately follows the phase detector, was integrating a different number of phase detected cycles for the upstream than for the downstream transmissions. This was because the number cycles occurring within 1.25 millisecond period would change in a manner proportional to the beat frequency. The magnitude of the error would be zero when the beat frequency was at a null and would increase in magnitude as the beat frequency increased until a new null would be reached at the next multiple harmonic and the cycle would repeat.
This invention improves the patented system by eliminating the free running constant frequency clock and by utilizing a divided down submultiple of the frequency of the VCO for all timing signals used in the flowmeter system. Thus, with a fixed number of phase detected signals, all problems of offset due to the use of a separate time base clock have been eliminated.
Accordingly, it is a primary object of this invention to provide a synchronous clock for the flowmeter system which eliminates offset errors introduced into the system by the combination of a free running constant frequency clock source and a variable frequency oscillator in the system.
Still another object of this invention, more specifically stated, is to provide a flowmeter system with a clock frequency as the basis for the system operation which is a divided down submultiple of the variable frequency voltage controlled oscillator in the system.